(1) Field of the Invention
This invention relates to the formation of halftone microimages and in particular to the use of halftone screens suitable for use in ultra-high resolution printing processes, wherein the halftone screens are formed by interferring coherent light beams.
(2) Description of the Prior Art
To reproduce an original continuous tone record such as a photograph or especially a microfilm record via conventional printing processes, it is first necessary to convert the original continuous tone record into a halftone print. In such a print, the record is subdivided into an array of evenly spaced dots or lines of various sizes in which larger dots or thicker lines represent the denser (darker) portions of the record, and thinner dots and lines represent the less dense (lighter) areas of the record. By varying the size of the dots or lines, a range of tone densities from very dark to very light can be obtained. The conversion of a continuous tone record into a halftone print useful in printing processes is an ancient art, going back at least to the sixteenth century, at which time, a continuous tone record was converted to a halftone print by carving the assembly of dots and/or lines into a block of wood or stone.
Since the advent of photography, many photomechanical processes have been described for making this conversion, wherein a halftone print of a continuous tone record is made by causing light from a record to pass through a screen having an array of dots or lines into a photosensitive material, which is then processed to provide the halftone print having dots or lines in graduated sizes corresponding to the various optical densities of the continuous tone record.
Generally, the halftone print or the printed replica is a one-to-one copy of the original continuous tone record, and is suitable for direct observation without enlargement. In such applications, a screen is used that has from about 2.5 to 8.0 lp/mm (line pairs per millimeter). Where the desired halftone print is a microimage, e.g., an image that is reduced in size, the use of screens having 8 lp/mm or even 20 lp/mm is not satisfactory, because when such microimages are viewed with enlargement of 20 to 30 times or more, the screened image is presented as an unacceptably coarse image.
U.S. Pat. No. 3,589,899 (Hersh) describes a method for optically converting a continuous tone image into a halftone image of reduced size by the steps of screening the original image with a screen having 200 lp/in. (8 lp/mm) and preferably simultaneously copying the image at a 2.4X reduction ratio, to produce a halftone image at about one-half the original size. The reduced halftone copy is then further optically reduced at a reduction ratio of 10 to 1 to obtain a microimage halftone print containing about 2000 lp/in. (80 lp/mm). Generally, microimages provided by such a process in which a screened halftone image is further reduced will exhibit a reduction in the number of grey level steps of contrast and in an accompanying loss of resolution. For example, a microimage halftone print so produced containing 80 lp/mm may exhibit approximately a 20 percent loss of grey level steps as a result of dot degradation during the imaging process when the microimage is subsequently reproduced in enlarged form.